KVM: x86/mmu: Harden "map private PFN" against unexpected root invalidation

Move kvm_tdp_mmu_map_private_pfn()'s reload of the MMU into its tight loop
so that an unexpected root invalidation has a better chance of being
handled gracefully, even though it should be impossible for the vCPU's root
to be invalidated after the initial reload.  As is, encountering an invalid
root is *guaranteed* to put the task into an infinite loop (albeit a
breakable loop that honors NEED_RESCHED).

Add a WARN to try and detect bugs that break KVM's expectations, along with
a comment to explain why it should be impossible for the root to be
invalidated.

Note, the loop in question doesn't actually check for a stale page fault,
i.e. likely won't detect an invalid loop in the first place.  That bug will
be addressed shortly.

Cc: Kai Huang <kai.huang@intel.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20260826164214.756512-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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